1. Field of the Invention:
This invention relates to a zoom lens, and more particularly to a focusing method for use in the type of zoom lens which has two components of negative and positive powers in this order from the front with the air separation therebetween being varied to effect zooming, or so-called two-component zoom lenses.
2. Description of the Prior Art
In the past, most of the zoom lenses have employed the so-called front focusing method which has the purpose of moving the frontmost lens component of the lens system to effect focusing. Also, the zoom lens in the form of comprising, from front to rear, a 1st component for focusing, a 2nd component for variation of the image magnification, a 3rd component for image shift compensation, and a 4th component for image formation, or the so-called 4-component zoom lens, provides stabilization of the position of the image plane against later actuation of any zooming after the in-focus condition has once been established on a certain object distance, regardless of what value it takes. On the other hand, the so-called two-component zoom lens employing the front focusing method tends to displace its image plane though very slightly, as zooming follows focusing. Since, however, the amount of this displacement is so small that no problem arises in actual practice, the front focusing method has found its use in many kinds of zoom lenses.
The use of the front focusing method, however, leads to forward axial movement of the front component as focusing is effected down to shorter object distances, with the result that aberrations are rapidly increased, and the illumination of the corners of the picture frame is appreciably lowered. To avoid this, the outer diameter of the front lens members must be increased, and a complicated lens form must be taken. This makes the optical system bulky and heavy and further renders it impossible to achieve a much-desired shortening of the lower limit of the focusing range. Another drawback arising from the use of an automatic focusing device in the camera is that when the component movable for focusing is heavy and bulky, the load to the focusing drive system is large and the speed up to the establishment of the in-focus condition becomes slow.
For this reason, many proposals on the focusing method by moving the rear lens component which has a smaller diameter have been made.
As zoom lenses having the focusing provision at part of the rear lens component, mention may be made of those disclosed in Japanese Laid-Open Patent Application No. Sho 57-26816, and U.S. Pat. Nos. 3,391,973, 4,110,006, 4,145,115 and 4,231,636.